Fitbit is the dominant market leader in activity tracking. Personally, I do not place much value in Apple Health. My hunch is that FB chose not integrate with HealthKit because the whole Apple health app stack is a stinking turd-- FB is vastly superior. The top two, in my humble opinion, are the FB Charge HR and FB Surge, depending on whether you want the GPS and more robust features of the Surge.
FB's strengths are its user base; mature activity, health, and fitness app platform; and social networking. No one else is even remotely close. The activity tracking hardware from everyone (including the AW) is largely a commodity item. If the activity tracking is important, then you do not want to undervalue the importance of the app and user platform.
I have an AW and I really like it. But, I use FB for activity tracking and health. Apple has a few years of catch-up work before I will give up my FB universe.